darTZeel Audio NHB-108 - Manual (Page 9)

darTZeel NHB-108 model one Audiophile's technical manual Page 9 of 28
5. The audio electronics
Simplicity.
The whole darTZeel NHB-108 model one
electronic concept could come down to
this single word.
Simplicity does not mean simple, how-
ever.
Purity
Total cancellation or absence of har-
monic distortion does not mean "purity",
alas.
You certainly know that most musical
instruments are very rich, harmonically
speaking. Thanks to these harmonics, we
can distinguish between a saxophone
and a flute.
Did you know, though, that the level of
these natural harmonics can easily reach
20 to 40% of the fundamental note, de-
pending on the instrument played? But
do we say that an instrument distorts?
5.1. What is distortion?
In the audio world, any signal change is
called "distortion". A lot of different kinds
of distortion exist, but we mostly speak
about "Total Harmonic Distortion", or
THD. THD is so well known that we more
than often forget that other ones do ex-
ist.
Negative Feed-Back, NBF, is a smart
electronic trick used for diminishing or
even eliminating all type of distortions,
and can be roughly described as follows:
At the amplifier input we put a signal
having an inverse deformation from the
one it naturally produces at its output.
So, the new output signal will be "puri-
fied" since the 2 inverse deformations
will cancel each other.
This is what the theory says.
Now let us see what actually happens in
practice. We will discuss only the best-
known distortions, given that their be-
havior is simpler to explain.
5.1.1. Keeping in harmony
Let us take an amplifier especially de-
signed to produce exactly 1% of THD.
Then let us feed it with a recorded musi-
cal instrument. We shall suppose that
this instrument is moderately rich har-
monically, say around 20%.
Now, let’s be a bit optimistic and sup-
pose that the speakers we will use are
truly perfect, without a single trace of
any distortion.
Well, now, let us try figure the THD we
will have at the output of the amplifier.
We could be tempted to say "21%, since
20 plus 1 makes 21, no?"
Okay… So then, which "golden ears"
could notice the THD difference between
20 and 21%? Who could swear that the
amplifier does add distortion?
In fact the distortion change is even less
than this. THD, like most other non-
correlated physical phenomena, does not
increase in a linear fashion. The "total"
THD is equal to the square root of the
sum of the squared individual THDs.
A little equation is even better than a
less-than-clear definition:
( ) ( ) ( )
2 2
2
2
1
....
n
HD HD HD THD
+ + + =
In our particular case, we only have 2
terms, so the equation becomes:
% 025 . 20
100
1
100
20
100
2 2
= 





+ 





=
THD
And now? Who could distinguish between
a 20.000% harmonically rich musical
instrument, and another one producing
20.025%? Not us for sure.
And the very next note, how rich is it?
And the next one? Oh! The song has al-
ready finished? What sort of music was
it?...
Even though in reality THD is a bit more
complicated than this, we can still see
that a posteriori, it does not have the
importance that was attached to it for
decades – quite the contrary.
If at darTZeel we could completely elimi-
nate THD while maintaining our 3 main
criteria, then maybe we would do so.
Maybe. Just for the beauty of the ges-
ture.